This invention relates to printing inks and coating compositions. More particularly it relates to printing inks and coating compositions wherein the resin is a 100-percent solids polyester.
Ideal printing inks and overprint coatings have a specific combination of properties that include good gloss, hardness, resistance to pasteurization and chemicals, and the like; these properties previously have been achieved with conventional high molecular weight solvent-based resins. Environmental legislation and the decreased availability of petroleum-based materials, however, have made it essential to eliminate, or at least to severely restrict, solvents in inks and coatings. Accordingly, there has been an increasing need for resin and alkyd systems that are solvent-free and that retain the high performance characteristics of the solvent-based resins.
High-solids polyesters for use in printing inks and coating compositions are known. U.S. Pat. No. 4,040,995, for example, teaches hydroxyl-functional oil-modified polyesters. These resins, the products of the reaction of a diol, a polyhydric alcohol having at least four hydroxyl groups, a dicarboxylic acid, and a saturated oil or fatty acid derived therefrom do not meet the required high performance standards with regard to their properties of pigment wetting, rheology, gloss, chemical resistance, and cure rate with the crosslinking agents used in inks and varnishes.
Hydroxy-functional oil-free polyester resins are disclosed in U.S. Pat. No. 4,113,793. These resins, the products of the reaction of a diol, a polyhydric alcohol having at least three hydroxyl groups, and isophthalic acid likewise fail to meet the high performance requirements with regard to the properties of pigment wetting, cure rate with crosslinking resins, rheology, gloss, and chemical resistance.
In U.S. Pat. No. 4,181,638, there is disclosed very low molecular weight polyesters derived from an aliphatic diol, a cyclic dicarboxylic acid, and a monobasic fatty acid. These materials, which have virtually no acid functionality and low hydroxy functionality, do not adequately wet pigments and give films of insufficient hardness and poor chemical resistance.
It has been determined that suitable resins for printing inks and overprint coatings should be free of volatile solvents. They should have Gardner viscosities of Z.sub.4 to &gt;Z.sub.10 in order that the ink have satisfactory flow; too high a viscosity gives poor tack stability and hence poor transfer of the ink; too low a viscosity results in misting of the ink. The resins should have high acid numbers, generally within the range of about 20 to 90, in order to wet pigments properly and effect a satisfactory cure with crosslinking resins; also they should have high hydroxyl values, usually about 20 to about 400, in order to provide sufficient reaction sites with crosslinking resins so as to obtain cured materials which have a high crosslink density and are resistant to solvents.